We have described in U.S. Pat. No. 3,823,424 a knee mechanism comprising thigh link means for attachment to upper portions of the leg, shin link means for attachment to a shin of the leg, and anterior and posterior link means pivoted between the thigh and shin links, with:
(a) the line joining the pivot between the anterior link means and the shin link means to the pivot between the posterior link means and the shin link means sloping rearwardly and downwardly;
(b) the anterior link means being shorter than the posterior link means;
(c) the pivots on the shin link means spaced apart by more than twice the distance separating the pivots on the thigh link means;
(d) the pivots on the thigh link means disposed at intermediate positions relative to the horizontal positions of the pivots on the shin link means when the leg is in a fully extended position; and
(e) the articulation defined by said links having an instantaneous centre of rotation constituted by the intersection point of a line drawn through the pivots of the anterior link means and a line drawn through the pivots of the posterior link means, said centre of rotation being located in substantially the region of that of a natural knee and lying upon a curve that is ascending through approximately 15.degree. of flexion of the knee mechanism from its fully extended position.
The resultant articulated joint provided a stable weight support until it had been flexed through the predetermined angle from the extended position, which was about 15.degree.. The practical joint was capable of flexure to about 120.degree., to permit a natural sitting posture. To this end both the forward and rearward links were cranked rearwardly from about their mid points to avoid mutual interference. Alteration of the lengths of the links and of their pivot points could be made to adapt the characteristics of the joint to suit the individual stump condition of a patient, but it was found that the angulation between lines intersecting the pivot axes of the links was critical. The particular combination of dimensions found to be important was that the line joining the thigh and shin pivots of the anterior link sloped forwardly and downwardly at an angle of 41.degree. from the vertical and that the included angles in an irregular quadrilateral defined by the pivot positions of the unflexed joint were 146.degree. at the thigh pivot of the anterior link, 49.degree. at the shin pivot of the anterior link, 82.degree. at the thigh pivot of the posterior link and 53.degree. at the shin pivot of the posterior link. The stated angles were found in practice to be absolutely vital. In an experiment, moving the pivot points one at a time by a mere 1/16 inch (equivalent to an average change of included angle of about 0.5.degree.) was found to produce quite unacceptable loss of function. Gaping between the knee part and the front upper edge of the shin part occurred, the maximum degree of flexion was reduced and there was a tendency for angular movement to become locked.
Extensions of the four bar knee idea e.g. to include swing phase control devices are described in our Patent Specification Nos GB-A-1536007 and GB-B-2134392 and for use in wooden knees are described in our Patent Specification GB-A-1546126. The same geometry was illustrated by Robert Kellie & Son in Patent Specification No. 13003738. We have used the four bar knee widely in a range of different devices, but in each instance we have preserved the above geometry because of its critical nature.
A swing phase control mechanism is described in Specification No. GB-A-1536007 and an extension bias mechanism is described in Specification No. GB-B-2134392. A locking mechanism for a mechanism of the four bar knee is described in Specification No. GB-B-2134392.